The invention concerns metal phosphates.
Rechargeable Li-ion accumulators are wide-spread energy storage means, particularly in the field of mobile electronics. Lithium metal oxides such as for example LiCoO2, LiNiO2, LiNi1-xCoxO2 and LiMn2O4 have established themselves as cathode materials. Besides the oxides, lithium-bearing phosphates with an olivine structure such as for example LiFePO4 (LFP) have also been developed, which are suitable as cathode materials. Those materials are distinguished by good power output, high specific capacitance and very high stability.
Besides LFP there are further lithium-bearing phosphates which are discussed as commercially usable cathode materials such as for example LiMnPO4, LiCoPO4 or LiNiPO4. In addition mixed-metal compounds of the type LiAxByCzPO4 ((x+y+z)=1) are also discussed such as for example alloys of LiNiPO4 and LiCoPO4 in the form of LiNixCox-1PO4 or LiFexMn1-xPO4.
In particular LiFexMnyPO4 and LiFexMnyMzPO4 (LFMP), wherein M is a metal cation like for example Mg are discussed as suitable compounds for replacing pure LiFePO4 (LFP) in cathode materials. Because of the higher working voltage of manganese- or nickel- or cobalt-bearing compounds in relation to iron-bearing olivines it is possible to achieve a higher level of energy storage density.
DE 10 2009 001 204 describes a process for producing crystalline iron(III) orthophosphate dihydrate (FOP) with phosphosiderite or metastrengite II-crystal structure which by virtue of the production process and the material properties is highly suitable as a precursor compound for the production of LFP in accordance with processes described in the literature.
WO 97/40541, U.S. Pat. No. 5,910,382 and WO 00/60680 describe the production of lithium mixed-metal phosphates, wherein generally firstly physical mixtures of various metal salts or also metallorganic compounds are produced, which in a subsequent step are calcined with conventional methods of solid-body synthesis at high temperatures and possibly with atmosphere control. In most cases in that respect the starting compounds are broken down in such a way that only the desired ions for construction of the target compound remain in the reaction system.
To achieve ideal isotropic distribution of the various cations in a crystal matrix, generally, in thermal processes as in calcination, a sufficiently high level of energy must be introduced into the reaction system to ensure efficient ion diffusion. In general intensive mixing of all the raw materials used is carried out as a preliminary step to reduce the amount of energy and time involved. In particular dry- or wet-mechanical processes, for example ball mills, are suitable for mixing the raw materials. That however results in mechanical mixtures of particles or crystals of various metal salts. In the subsequent calcination operation it is therefore necessary to ensure that the ions necessary for constructing the desired crystal phase diffuse beyond the primary crystal grain boundaries. Temperatures over 700 to 800° C. and calcination times over 15 hours are usually required for that purpose. It is also usual for the physical mixtures to be initially subjected to a heating step at lower temperatures (300-400° C.) to bring about initial breakdown. Those intermediate products are then comminuted once again and intensively thoroughly mixed in order to achieve good results overall in the sense of phase purity, crystallinity and homogeneity. The known thermal processes are therefore energy- and time-intensive.
In addition particularly high purity demands are made on the starting materials used for the production of cathode materials for lithium-ion batteries as all constituents and impurities which do not break down remain in the reaction system and thus in the product. Upon the breakdown of cations and anions of the metal compounds used as starting materials (for example NH4+, C2O42−, (CH3)(CH2)nCOO−, CO32−, etc), gases are also produced, which must be treated in the exhaust gas flow in an expensive and complicated procedure because of potentially dangerous properties (for example CO, NH3, NOx, etc).
CA 02443725 describes the production of LiXYPO4 (X, Y=metal, for example Fe, etc) using iron sulphate, manganese sulphate and lithium phosphate and additionally lithium hydroxide as starting materials, from which firstly a solid substance mixture which is not characterised in fuller detail is produced, which is then converted into the desired product by a calcination step at 300 to 1000° C.
Introducing given metals in the form of their sulphates in an equimolar amount in relation to the phosphate usually requires the product to be subjected to an intensive washing process to reduce the sulphate content to a tolerable amount. By virtue of the corrosive action it is known that sulphate is an unwanted impurity in lithium-ion batteries. Due to an intensive washing process however lithium can also be removed from the product in a considerable amount as only trilithium orthophosphate has a very low level of solubility among the lithium orthophosphates. If the product in CA 02443725 is subjected to such a washing process, it is to be assumed that lithium is washed out. However CA 02443725 does not mention a washing process, which in turn would result in a high level of sulphate contamination in the product.
In principle it is possible to achieve quite homogeneous cation distribution levels by hydro- or solvothermal processes if the levels of solubility and complexing constants or the crystalline growth factors of the introduced cations and anions can be so controlled and adjusted by way of the reaction procedure in the selected matrix that only the desired species is produced in isolatable form. In many cases surface-active substances or also additives which promote the formation of a given crystal phase or growth in a preferred direction, so-called templates which are known to the man skilled in the art, are used here to control crystal growth. In those processes, operation is often implemented in closed systems beyond the boiling point of the reaction matrix, whereby very high pressures are involved. That places high demands on the reactor technology. In many cases the products obtained have to be nonetheless or additionally subsequently calcined to ensure the necessary crystallinity. The surface-active additives must be quantitatively removed in order not to cause any negative influences in the subsequent use. That is also achieved by heating, in which case those substances burn or char/soot.
Processes operating in a pressure-less mode are also described, wherein the crystallisation times of the desired products are always specified as being several days to weeks. That casts doubt on profitability in commercial use.